As it turns out, it wasn't the inherit bit (as tye suggested) or a child-inheritance problem at all. Win32 by default won't let processes created with ProcessCreate() inherit sockets in a straightforward way, they have to be passed externally through other means.

Through suggestions given by others reading this node, I tried isolating exactly when the sockets started getting ignored. So I tried POD'ing out all of the shutdown routines. And it was this bit of code here near line 244 in testcase.pl:

IO::Select->remove() on an already-closed descriptor seems to alter another descriptor in unknown ways. So it turns out that the file descriptor being affected by remove and the one being closed in the above example *aren't* the same descriptor. The simple fix is:

Which solves the problem nicely. The earlier opened descriptors stay open like they should.

I'm unclear as to whether that's a bug in Perl or IO::Select. There are open bugs in Win32 Perl having to do with file descriptor mixups during process creation, so I wouldn't doubt that this might also be another one.